Accessing image texture connected to Cycles material Diffuse BSDF
thanks to zeffii :

    diffuse_BSDF = mtl.node_tree.nodes.get('Diffuse BSDF')

    if diffuse_BSDF != None:
        # grab color input
        input = diffuse_BSDF.inputs[0]

        if input.is_linked:
            src = input.links[0].from_node
            if src.type == 'TEX_IMAGE':
                self.File.Write('tex_src:{}\n'.format(src.image.filepath))
            elif src.type == 'MIX_RGB':
                for i in src.inputs:
                    if i.type == 'RGBA':
                        if i.is_linked:
                            src_i = i.links[0].from_node
                            if src_i.type == 'TEX_IMAGE':
                                self.File.Write('tex_src:{}\n'.format(src_i.image.filepath))


(obviously the write function is script specific but maybe that's helpful to others)